Thrombin is a powerful factor in regulating the state of the cardiovascular system. It is clear that thrombin is an integral component involved in the formation of blood clots by catalyzing the conversion of fibrinogen to fibrin, which is an integral pan of most clots. In addition, thrombin is known to act directly on cells in the blood and in the interior blood vessel wall, and specifically to activate platelets to form clots. Thrombin-induced platelet activation is particularly important for arterial thrombus formation, a process that causes myocardial infarction and some forms of unstable angina and stroke. In addition, thrombin promotes intimation and other cellular activities. Thrombin is chemotactic for monocytes, mitogenic for lymphocytes, and causes endothelial cells to express the neutrophil adhesive protein GMP-140 on their surfaces and inhibits the growth of these cells. Thrombin elicits platelet-derived growth factor from the endothelium and is a mitogen for mesenchymal cells.
Because thrombin is capable of direct activation of cells, it is assumed that at least one thrombin receptor exists. However, it has not been possible to detect the presence of a thrombin receptor by traditional binding studies, since thrombin is capable of binding a large number of sites present on cells which do not directly mediate the cellular responses to thrombin, and thus the background levels of binding are prohibitively high.
The thrombin-binding proteins that have been identified do not seem to function as transduction molecules (Gronke, R. S. et al., J Biol Chem (1987) 262:3030-3036; Okamura, T., et al., J Biol Chem (1978) 253:3435). Modified thrombins that are physiologically inactive seem to bind to platelets in the same way as thrombin itself. Thus, the binding sites identified by traditional binding studies may not be related to functional thrombin receptors. Also, since thrombin is a protease, if the receptor were proteolytically cleaved by its interaction with thrombin, the receptor's ability to bind tightly to thrombin would be decreased. All of the foregoing factors suggest that traditional binding studies in an effort to identify and characterize a thrombin receptor might ultimately be unproductive.
While it has been assumed that a thrombin receptor exists, no direct experimental evidence exists from the studies conducted so far, whether proteolytic cleavage by thrombin is involved in its receptor activation. When thrombin is treated with reagents which covalently modify and render it proteolytically inactive, its ability to stimulate platelets is abolished (Bemdt, M. C., et al., "Platelets in Biology and Pathology" (1981) Elsevier/North Holland Biomedical Press, pp. 43-74; Martin, B. M., et al., Biochemistry (1975) 14:1308-1314; Tollefsen, D. M., et al., J Biol Chem (1974) 249:2646-2651; Phillips, D. R., Thrombin Diath Haemorrh (1974) 32:207-215; Workman, E. F., et al., J. Biol Chem (1977) 252:7118-7123; Greco, N. J., et al., Blood (1990) 75:1983-1990). The modified forms of thrombin described in the reports above contain bulky or charged moieties that occupy the active site and also obscure additional regions of the surface of thrombin that bind substrate (Bode, W., et al., Embo J (1989) 8:3467-3475). Some of the chemically-modified thrombins do not, in fact, block thrombin-induced platelet activation.
Coughlin, et al., WO 9214750, describes cloning and sequencing of DNA encoding the cell surface receptor for thrombin, and recombinant production of the thrombin receptor at cell surfaces. Assay systems for the detection of thrombin and the evaluation of thrombin agonists and antagonists are described.